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# Simple 5 minute Tutorial

## Overview

> The `ruffus` module is a lightweight way to add support for running computational pipelines.

> Computational pipelines are often conceptually quite simple, especially if we breakdown the process into simple stages, or separate tasks.

> Each task is represented by a python function

## A Simple example

### @follows

The @follows(. . .) python decorator indicates the order in which tasks should be run:

    from ruffus import *

    def first_task():
        print "First task"

    @follows(first_task)
    def second_task():
        print "Second task"

the `@follows` decorators indicate that the `first_task` function precedes `second_task` in the pipeline.

### Running

> Now we can run the pipeline by:

>     >>> pipeline_run([second_task])

> Which gives:

>     Task = first_task
>     First task
>         Job completed
>     Task = second_task
>     Second task
>         Job completed

> Because `second_task` depends on `first_task` , both functions will be executed in order.

### @files

The @files(. . .) decorator provides parameters to a task. The task function is called in parallel with each set of parameters.

(We describe each task function call as a separate job.)

The first two parameters of each job are the input and output files (respectively). A job will be run only if the file timestamps are out of date.

Let us add i/o parameters to the previous python code:

    from ruffus import *
    import time

    #---------------------------------------------------------------
    #
    #   first task
    #
    task1_param = [
                        [ None, 'a.1'], # 1st job
                        [ None, 'b.1'], # 2nd job
                  ]

    @files(task1_param)
    def first_task(no_input_file, output_file):
        open(output_file, "w")

        # pretend we have worked hard
        time.sleep(1)

    #---------------------------------------------------------------
    #
    #   second task
    #
    task2_param = [
                        [ 'a.1', "a.2", "    1st_job"], # 1st job
                        [ 'b.1', "b.2", "    2nd_job"], # 2nd job
                  ]

    @follows(first_task)
    @files(task2_param)
    def second_task(input_file, output_file, extra_parameter):
        open(output_file, "w")
        print extra_parameter

    #---------------------------------------------------------------
    #
    #       Run
    #
    pipeline_run([second_task])

Gives:

    Task = first_task
        Job = [null -> "a.1"] completed
        Job = [null -> "b.1"] completed
    Task = second_task
        1st_job
        Job = ["a.1" -> "a.2", "1st_job"] completed
        2nd_job
        Job = ["b.1" -> "b.2", "2nd_job"] completed

If you ran the same code a second time, nothing would happen because `a.2` is more recent than `a.1` and `b.2` is more recent than `b.1` .

However, if you subsequently modified `a.1` again:

    >>> open("a.1", "w")

You would see the following:

    >>> pipeline_run([second_task])
    Task = second_task
        1st_job
        Job = ["a.1" -> "a.2", "    1st_job"] completed
        Job = ["b.1" -> "b.2", "    2nd_job"] unnecessary: already up to date

The 2nd job is up to date and will be skipped.

### Displaying

> We can see a flowchart of our fledgling pipeline by executing:

>     pipeline_printout_graph ( open("flowchart.svg", "w"),
>                              "svg",
>                              [second_task])

## More

See the full tutorial for more detail on:

> - Ordering tasks
> - Using files as parameters
> - Generating parameters on the fly
> - Multi Processing
> - Run jobs in parallel
> - Handling errors and exceptions
> - Generating parameters using regular expressions
> - Manual dependency checking
> - Signalling the completion of each task
> - Logging messages

### Navigation

- index
- modules |
- next |
- previous |
Ruffus v. 1.0.8
- Home |
- Contents |
- Cheat sheet |
- Full tutorial |
- FAQ |
- Future plans |
- Search »